The Principle of SA8281 SPWM Wave Generator and Its Application in Frequency Converter

1 Introduction
Pulse width modulation technology controls the on and off of switching elements in a certain pattern to obtain a set of rectangular pulse waveforms with equal amplitude but different widths, which is used to approximate a sinusoidal voltage waveform. The application of pulse width modulation technology in inverters has greatly promoted the development of modern power electronics technology and modern speed control systems.
In recent years, due to the continuous emergence of field-controlled self-shutoff devices, the corresponding high-frequency SPWM (sinusoidal pulse width modulation) technology has been widely used in motor speed control. SA8281 is an integrated circuit for three-phase SPWM wave generation and control launched by MITEL. It is easy to interface with the microprocessor, with built-in waveform ROM and corresponding control logic. After setting, it can independently generate three-phase PWM waveforms. Only when the output frequency or amplitude needs to be changed, the microprocessor needs to intervene. The microprocessor only takes a short time to control it, so it is capable of detecting, protecting and controlling the entire system. The inverter based on SA8281 and 89C52 has the advantages of simple circuit, complete functions, high performance-price ratio and good reliability.

2 Main features and pin functions of SA8281
2.1 Main features of SA8281
SA8281 has flexible interface with microprocessor, suitable for Intel and Motorola bus interfaces, and simple and convenient programming control; SA8281 has 6 standard TTL level outputs to drive 6 power switch devices of the inverter. The operating parameters such as carrier frequency, modulation frequency, modulation ratio, minimum pulse width, dead time, etc. can be set directly through software, with flexible settings, no external circuit is required, and hardware costs are saved; the operating frequency
range is wide and the accuracy is high, the triangular carrier frequency is adjustable, when the clock frequency is 12.5 MHz, the carrier frequency can reach up to 24kHz, and the output modulation frequency can reach up to 4 kHz, and the fully digital pulse output has high accuracy and temperature stability;
when the circuit remains unchanged, the performance indicators of the inverter can be changed by modifying the control parameters, driving different loads or working under different working conditions;
the forward and reverse rotation of the motor can be achieved by changing the phase sequence of the SPWM pulse;
the independent blocking terminal can instantly block the output SPWM pulse, which can handle the occurrence of sudden situations of the motor.
2.2 Pin Functions of SA8281
SA8281 uses 28-pin DIP and SOIC packages, and its pin arrangement is shown in Figure 1. There are two main types of pins: one is the interface and control pins with the microprocessor; the other is the SPWM pulse output and control pins.

(1) Interface and control pins AD0_AD7 with the microprocessor: data and address multiplexing bus. CS, WR, RD, ALE are chip select, write, read and address latch signal lines respectively.
(2) SPWM pulse output and control pins
RPHB, YPHB, BPHB control the lower arm switch tubes of R, Y, and B respectively through the drive circuit.
RPHT, YPHT, BPHT control the upper arm switch tubes of R, Y, and B respectively through the drive circuit.
SET TRIP: This pin can quickly turn off all SP-WM signal outputs, and high level is valid.
TRIP: Output lock state. When SET TRTP is valid, TRIP is low level, indicating that the output has been locked.
ZPPR: Output modulation wave frequency.
WSS: Output sampling waveform.
(3) Other pins
RST: Hardware reset pin, low level is valid.
CLK: Clock input terminal.
VDD, VSS: Positive and negative power supply terminals.

3 Internal structure and working principle of SA8281
The internal structure of SA8281 is shown in Figure 2.

As can be seen from the internal structure diagram, the data from the microprocessor enters the initialization register and control register through bus control and decoding, controls the phase and control logic circuits, and realizes the system parameter setting. The external clock input sets the frequency separately through the frequency divider. SA8281 reads the waveform data directly from the waveform ROM according to the signal of the address generator, and then composes it into a complete SPWM waveform of 0°~360° through the phase control logic. The whole process does not require the control of the microprocessor. Each phase output control circuit is composed of a pulse width deletion and a pulse delay circuit. The SPWM wave deletes the narrower pulses with a pulse width less than the deletion time through the pulse width deletion circuit. The delay circuit generates a dead time to ensure that the two switch tubes on any bridge arm will not be turned on together at the moment of conversion.

4 Inverter Design
The overall structure of the inverter is shown in Figure 3. The three-phase AC voltage is rectified into a 514 V DC voltage by an uncontrolled rectifier bridge and sent to the DC link. After capacitor filtering, a relatively flat DC voltage is obtained. The 89C52 8-bit microprocessor controls the dedicated three-phase PWM circuit SA8281 to generate the required SPWM signal and control the inverter bridge composed of IGBT to work in SPWM mode. The interface circuit schematic diagram of 89C52 and SA8281 is shown in Figure 4.

As shown in Figure 4, the address data bus of SA8281 is directly connected to the PO port of 89C52, and the three control lines, WR, RD, and ALE, are respectively connected to the corresponding pins of 89C52, and the chip select signal CS is connected to P2.7. The microprocessor's Pl.0 controls the reset pin RST of SA8281. Considering that the 89C52 microcontroller does not have a non-maskable interrupt, all fault signals are merged and sent directly to the SET TRIP pin of SA8281 during the design to achieve fast locking when there is a fault, and TRIP is used to generate an interrupt to handle and recover the fault in the interrupt service program. In order to avoid false locking, each fault signal is added with a filter delay circuit, and the merged fault signal is further passed through a narrow pulse elimination circuit composed of a monostable circuit to eliminate the influence of interference pulses.
After the carrier frequency, modulation frequency, modulation ratio, minimum pulse width, dead time and other working parameters are set by software, the microprocessor intervention is required only when the output frequency or amplitude needs to be changed. The microprocessor only takes a short time to control it. Therefore, the main task of the microprocessor is to ensure that the power device operates under normal working conditions. When an abnormal situation occurs, it can detect the fault in time and lock the system output, cut off the main circuit power supply, stop the system from working, and ensure that the power device is not damaged. The designed inverter protection functions include over-current protection, over-voltage protection, under-voltage protection, over-heat protection and short-circuit protection.

5 Software Implementation
Software implementation is the core of the entire inverter control. It determines the output characteristics of the inverter, such as voltage, frequency range and stability, harmonic content, protection function implementation, system reliability improvement, etc. Figure 5 shows the program flow of this system. The program adopts modular design. After the microprocessor completes initialization, it calls the keyboard display subroutine to first determine whether to start. If started, SA8281 is initialized and SPWM pulse output is allowed. The inverter adopts soft start mode. The parameter calculation and setting of the control command are mainly used to determine the frequency adjustment range, dead time, output voltage amplitude, center frequency, etc. When the parameters are set, the microprocessor mainly monitors the working status of the inverter and makes corresponding processing according to different fault conditions.

6 Conclusion
The use of SA8281 three-phase PWM waveform generator greatly simplifies the control circuit, reduces the number of components, makes the structure compact, reduces the cost, and improves the reliability. At the same time, SA8281 has a simple interface with the microprocessor, is easy to program, and has a high precision and temperature stability in the fully digital pulse output. It also has a wide operating frequency range and high output frequency resolution. In particular, when the output frequency and amplitude remain unchanged, there is no need for microprocessor intervention, and the CPU takes up less time. It is the preferred solution for low-cost inverters.